Infrared photography is an extraordinarily valuable technique for detecting preparatory underdrawings in paintings where the underdrawing is executed over a white priming with a medium containing carbon, such as ink, charcoal, or paint made with a carbon pigment—bone black, for example. Infrared radiation, invisible to the human eye, can penetrate a painting’s surface layers and capture an image with infrared-sensitive film or via specialized software on a computer screen.
Infrared photography and modified digital cameras work in the near-infrared waveband, with limited penetration (700–1000 nanometers). Pigments such as azurite and malachite block penetration at this wavelength, which is a major limitation since these pigments are used widely for robes, skies, and landscapes in medieval and Renaissance paintings. However, a technique known as infrared reflectography, developed by J. R. J. van Asperen de Boer in the late 1960s at the University of Utrecht, Netherlands, can penetrate much further, to 2500 nm. Unfortunately, the technique had low resolution and was labor-intensive. The arrival of solid-state digital technology in the mid-1990s transformed the technique. Experience with this technology shows that the waveband of 700 to 1700 nm provides an excellent image and can penetrate layers of azurite and malachite. Given that the waveband at which infrared images are taken will determine what might be revealed, it is important to know the waveband used so that accurate assessments and comparisons of underdrawings in art works can be made.